This invention relates to a system including a Stirling engine which utilizes heat at two temperatures, and more particularly to a Stirling engine having a high temperature heat exchanger which provides heat to the thermodynamic cycle at a high temperature, and an additional heat exchanger which accepts waste heat from another process or from the high temperature heat exchanger exhaust to increase the energy input into the engine.
The potential efficiencies and low emissions of the Stirling engine have inspired renewed interest and considerable development efforts in this technology. However, some development programs of great magnitude have been abandoned, in part because of the fundamental difficulty of power density. The power output of the Stirling engine when compared to its weight and cost has always been low compared to the conventional internal combustion engine. Since the Stirling engine utilizes a closed cycle for the working fluid, it is necessary that the structure for adding heat at high temperature and rejecting heat at low temperature be very efficient. However, there are certain basic limitations to the efficiency of the heat exchangers and regenerators in a Stirling engine. In order to achieve good heat exchange, it is necessary that the working fluid be exposed to a large surface area of the heat exchanger. This results, in practice, that high viscous losses are incurred in the heat exchangers, and that the heat exchangers and regenerator are relatively heavy. In addition, the necessity of adding large amounts of heat at high temperature through the primary heat exchanger imposes extremely stringent demands on the materials involved. The problem of adding high temperature heat in large quantities through a fine heat exchanger is in part the difficulty of regulating the temperature so that the temperature limit of the materials of the heat exchanger are not exceeded in localized areas.
Another difficulty encountered in the use of Stirling engines is that of efficiently utilizing all of the fuel energy in the burner so that energy is not wasted in high temperature burner exhaust. In practice, this has been most effectively achieved in a combustor preheater in which the burner exhaust gases are used to preheat the intake air so that the air for combustion arrives at the burner nozzles at a relatively high temperature and therefore less fuel is required to achieve the desired temperature of the combustion products. Although this may be a satisfactory solution in terms of laboratory results, it is not a satisfactory commercial solution because the preheater is bulky, heavy, complicated and expensive.
The failure of prior development programs to develop a commercially viable Stirling engine for most prime mover applications in spite of the intense efforts which the art has been exerting to find a solution to the problems, illustrates the difficulty of these fundamental problems. A solution to both of these problems in a single engine configuration would be a welcome development indeed.